1. Field of the Invention
The invention relates to troffer-style lighting fixtures, and more particularly, to indirect troffer-style lighting fixtures utilizing solid state light sources.
2. Description of the Related Art
Troffer-style fixtures are ubiquitous in commercial office and industrial spaces throughout the world. In many instances these troffers house elongated fluorescent light bulbs that span the length of the troffer. Troffers may be mounted to or suspended from ceilings, such as being suspended by a “T-grid”. Often the troffer may be recessed into the ceiling, with the back side of the troffer (i.e. troffer pan) protruding into the plenum area above the ceiling a distance of up to six inches or more. In other arrangements, elements of the troffer on the back side dissipate heat generated by the light source into the plenum where air can be circulated to facilitate the cooling mechanism. U.S. Pat. No. 5,823,663 to Bell, et al. and U.S. Pat. No. 6,210,025 to Schmidt, et al. are examples of typical troffer-style fixtures. These fixtures can require a significant amount of ceiling space to operate properly. Many of these fixtures utilize a grid of louvers to more evenly distribute the light and reduce glare.
More recently, with the advent of the efficient solid state lighting sources, these troffers have been used with solid state light sources, such as light emitting diodes (LEDs). LEDs are solid state devices that convert electric energy to light and generally comprise one or more active regions of semiconductor material interposed between oppositely doped semiconductor layers. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is produced in the active region and emitted from surfaces of the LED.
LEDs have certain characteristics that make them desirable for many lighting applications that were previously the realm of incandescent or fluorescent lights. Incandescent lights are very energy-inefficient light sources with approximately ninety percent of the electricity they consume being released as heat rather than light. Fluorescent light bulbs are more energy efficient than incandescent light bulbs by a factor of about 10, but are still relatively inefficient. LEDs by contrast, can emit the same luminous flux as incandescent and fluorescent lights using a fraction of the energy.
In addition, LEDs can have a significantly longer operational lifetime. Incandescent light bulbs have relatively short lifetimes, with some having a lifetime in the range of about 750-1000 hours. Fluorescent bulbs can also have lifetimes longer than incandescent bulbs such as in the range of approximately 10,000-20,000 hours, but provide less desirable color emission. In comparison, LEDs can have lifetimes between 50,000 and 70,000 hours. The increased efficiency and extended lifetime of LEDs is attractive to many lighting suppliers and has resulted in LED light sources being used in place of conventional lighting in many different applications. It is predicted that further improvements will result in their general acceptance in more and more lighting applications. An increase in the adoption of LEDs in place of incandescent or fluorescent lighting would result in increased lighting efficiency and significant energy saving.
LED components or lamps have been developed that comprise an array of multiple LED packages mounted to a (PCB), substrate or submount. The array of LED packages can comprise groups of LED packages emitting different colors, and specular reflector systems to reflect light emitted by the LED chips. Some of these LED components are arranged to produce a white light combination of the light emitted by the different LED chips.
In order to generate a desired output color, it is sometimes necessary to mix colors of light which are more easily produced using common semiconductor systems. Because of the physical arrangement of the various source elements, multicolor sources often cast shadows with color separation and provide an output with poor color uniformity. Thus, one challenge associated with multicolor light sources is good spatial color mixing over the entire range of viewing angles. One known approach to the problem of color mixing is to use a diffuser to scatter light from the various sources.
Many current luminaire designs utilize forward-facing LED components with a specular reflector disposed behind the LEDs. One design challenge associated with multi-source luminaires is blending the light from LED sources within the luminaire so that the individual sources are not visible to an observer. Heavily diffusive elements are also used to mix the color spectra from the various sources to achieve a uniform output color profile. To blend the sources and aid in color mixing, heavily diffusive exit windows have been used. However, transmission through such heavily diffusive materials causes significant optical loss.
Some recent designs have incorporated light sources or light engines utilizing an indirect lighting scheme in which the LEDs or other sources are aimed in a direction other than the intended emission direction. This may be done to encourage the light to interact with internal elements, such as diffusers, for example. One example of an indirect fixture can be found in U.S. Pat. No. 7,722,220 to van de Ven which is commonly assigned with the present application.
There have also been recent designs that focus more on retrofitting or redesigning existing troffer-style light fixtures in order to utilize LEDs as a light source. This allows manufacturers to use existing manufacturing capabilities to produce troffer housings for LEDs, which is thought to help in reducing overall troffer costs. In some of these fixtures, hundreds of LED packages are mounted to the surface of an existing troffer pan to essentially cover the troffer pan surface with emitters, sometimes utilizing up to 400 LED packages. The emitters are then driven with a relatively low electrical signal so that the fixture gives a relatively uniform output with no visible hot spots.
Troffer-style light fixtures are typically provided with a prismatic lens or diffuser over the troffer pan/housing opening that faces the room to be illuminated. The prismatic diffuser is included to disperse some of the light from the source. Despite the use of hundreds of LED packages in an effort to spread the light evenly, these LED fixtures can still exhibit multiple emission hot spots as the light passes through the prismatic diffuser, producing an undesirable output. These fixtures having hundreds of LED packages can be relatively expensive, with the bulk of the expense being the LED packages, along with the cost and complexity of mounting, interconnecting, and driving the LED packages.